Although communication carriers provide various bit-rate services with the spread of the Internet, maintenance and operation cost increases because a different transmission device is used in each bit-rate. There is a demand for unifying the transmission devices in order to prevent the cost increase. There has been proposed a transmission device that can deal with plural bit-rates to discriminate the bit-rate inside the device (Patent Document 1), and there has also been proposed a mechanism in which optical receiver sensitivity is varied according to the bit-rate (Patent Document 2).
A bit-rate discrimination circuit that discriminates the bit-rates is roughly classified into two type methods. In one of the methods, an edge portion (logical code switching portion of “0”/“1”) of a signal is output as a pulse having a constant time width, and the pulse is integrated with respect to the time to specify a signal switching frequency (that is, bit-rate) (Patent Document 1, hereinafter referred to as edge detection method). In the other method, the bit-rate is specified by detecting a low-frequency component of a consecutive identical signal included in the signal (Patent Document 2, hereinafter referred to as low-frequency detection method). Both the methods are based on the assumption that the communication is always conducted at the specified bit-rate after the bit-rate is specified, and the methods do not take into account the high-speed following of the constantly-changing bit-rate. This is because of the following reasons.
In the edge detection method, the edge portion of the signal is detected, and edge density is changed according to same-code continuation included in the signal. The edge density is obtained to some extent for an alternating signal even in the low bit-rate signal, while the edge density is reduced when many long continuous codes are included in the high bit-rate signal. Thus, in order to enhance discrimination accuracy, it is necessary to take some time to statistically make discrimination (usually, time of ten thousand bits to one million bits in terms of bit number). Therefore, an integration time of the edge signal is lengthened. In the low-frequency detection method, because the low-frequency component is detected as its name suggests, it is necessary to set a band of a lowpass filter to a low frequency. Therefore, a time constant of the lowpass filter becomes large order similarly to the integrator in the edge detection.
Recently, a point-to-multipoint network that deals with plural users with one station apparatus becomes widespread, and the need to accommodate users of different bit-rates arises with diversification of the transmission speed. However, for the above-described reasons, a function of instantaneously discriminating the bit-rate with respect to the signal whose bit-rate is switched at high speed is difficult to realized using the bit-rate discrimination circuit of Patent Document 1 or Patent Document 2.
A bit-rate discrimination mechanism that automatically discriminates and detects the received bit-rate is required in order to realize a multi-rate receiver that deals with the plural bit-rates. Furthermore, when the plural bit-rates to be dealt with do not have a relationship of an integral multiple or an integral fraction, a circuit and a device which are connected to a subsequent stage of the receiver must also be able to deal with the plural bit-rates that do not have the relationship of the integral multiple or integral fraction. Therefore, a signal discrimination circuit that can separate the signals according to the bit-rate of the input signal and output the signals from different output terminals in each bit-rate group having a relationship of the integral multiple or integral fraction or in each bit-rate is needed (for example, see Patent Document 3).
FIG. 25 is a block diagram illustrating a conventional receiver that deals with plural bit-rates by combining a signal discrimination circuit and a bit-rate discrimination apparatus automatically detecting received plural bit-rates.
The receiver in FIG. 25 automatically discriminates the bit-rates using the bit-rate discrimination circuit and the signal discrimination circuit, and individually outputs the desired bit-rates from different output ports. Specifically, the receiver includes a photo-detector 101, a trans-impedance amplifier (TIA) 102, a low bit-rate limiting amplifier (LA) 103, a high bit-rate limiting amplifier (LA) 104, a low-bit-rate-side control port 105, a gate circuit 106, a high-bit-rate-side control port 107, a gate circuit 108, a low bit-rate clock and data recovery circuit (CDR) 109, a high bit-rate clock and data recovery circuit (CDR) 110, a low bit-rate signal output terminal 111, a high bit-rate signal output terminal 112, a low-bit-rate discrimination circuit 113, a high-bit-rate discrimination circuit 114, and a reset-set flipflop circuit 115. The receiver in FIG. 25 outputs the received signal from the low bit-rate signal output terminal 111 and the high bit-rate signal output terminal 112 according to the high-bit-rate optical signal and low-bit-rate optical signal, which are input to the photo-detector 101. In the configuration of the receiver in FIG. 25, an output voltage of the high bit-rate limiting amplifier (LA) 104 is partially input to the low-bit-rate discrimination circuit 113 and the high-bit-rate discrimination circuit 114 to discriminate the bit-rates. In other words, the receiver in FIG. 25 has the configuration in which the bit-rates are discriminated by inputting the signals having the plural bit-rates from one input port to the respective bit-rate discrimination circuits.
Thus, the optical transmission receiving circuit can be formed using the bit-rate discrimination circuit and the signal discrimination circuit having the discrimination function such that the bit-rates are automatically discriminated to individually output the signals having the desired bit-rates from different output ports.